How do you get the key for the door in hidden lake in club penguin?

Go to the Pufflerescue Desk and play the black puffle. When you get the puffle on Level 1 wait for the squid and follow his shadow in order to find the door and get the key. Them exit the game and go to the door and unlock it.

P.S.
Make sure you pass over the bubbles so you can breathe. But if you don't you get smaller and smaller until you go, POOF!